1. Field of the Invention
The present invention relates to an image sensing apparatus and an imaging system.
2. Description of the Related Art
An image sensing apparatus such as a MOS sensor includes a pixel array in which a plurality of pixels are arrayed, a readout unit which reads out a signal from the pixel array and outputs a plurality of signals, and an output unit which obtains an image signal in accordance with the plurality of signals output from the readout unit and outputs it. The output unit performs correlated double sampling (to be abbreviated as CDS hereinafter) processing.
The readout unit reads out an optical signal and noise signal from the pixel of each column of the pixel array at different timings, and temporarily holds them in a line memory. The readout unit outputs the optical signal and noise signal held in the line memory to an optical signal output line (to be referred to as an S output line hereinafter) and a noise signal output line (to be referred to as an N output line hereinafter) in the output unit. The readout unit sequentially performs this operation for respective columns.
In the output unit, a difference circuit arranged at the succeeding stage of the S output line and N output line operates the difference between the optical signal transferred to the S output line and the noise signal transferred to the N output line (performs CDS processing), thereby sequentially obtaining image signals of the pixels of each column.
A case in which an optical signal and noise signal of the pixel of the first column are output to a first S output line and first N output line, and an optical signal and noise signal of the pixel of the second column are output to a second S output line and second N output line will be considered. In this case, a first difference circuit operates the difference between the optical signal output to the first S output line and the noise signal output to the first N output line (performs CDS processing), thereby obtaining an image signal of the pixel of the first column. A second difference circuit operates the difference between the optical signal output to the second S output line and the noise signal output to the second N output line (performs CDS processing), thereby obtaining an image signal of the pixel of the second column. With this arrangement, operations involved in the optical signals and noise signals of the first and second columns can be performed parallelly. This makes it possible to increase the operating speed of the output unit.
In this arrangement, the first S output line and first N output line correspond to the pixels of the first column, and the second S output line and second N output line correspond to the pixels of the second column. For this reason, it is a common practice to arrange the first S output line, first N output line, second S output line, and second N output line in this order. In this case, the first N output line is adjacent to the second S output line, so it is readily susceptible to crosstalk according to an optical signal by the second S output line.
To solve this problem, Japanese Patent Laid-Open No. 2004-153682 discloses an image sensing apparatus in which two pairs of N output lines and S output lines, i.e., a first S output line, first N output line, second N output line, and second S output line are arranged in this order. With this arrangement, the first N output line is adjacent to the second N output line, so it is susceptible to crosstalk according to a noise signal, a temporal change in level of which is smaller than that of an optical signal, by the second N output line. This makes it possible to reduce noise attributed to crosstalk on the first N output line, according to Japanese Patent Laid-Open No. 2004-153682.
In the arrangement disclosed in Japanese Patent Laid-Open No. 2004-153682, i.e., the arrangement in which the first S output line, first N output line, second N output line, and second S output line are arranged in this order, the smaller the spacings between the output lines, the shorter the distances between the first S output line and the second S output line and between the first N output line and the second S output line. This may increase crosstalk inflicted on the first S output line and first N output line by the second S output line. Furthermore, the distance between the first N output line and the second S output line is different from that between the first S output line and the second S output line. For this reason, noise attributed to crosstalk inflicted on the first N output line by the second S output line is more likely to be different from that attributed to crosstalk inflicted on the first S output line by the second S output line. In this case, even when the first difference circuit operates the difference between the signal output to the first S output line and that output to the first N output line, it is impossible to reduce noise attributed to crosstalk. That is, it is difficult to reduce noise attributed to crosstalk inflicted on the first S output line and first N output line by the second S output line.